Will Carpet Stifle a Radiant Slab?

Q.I'm finishing a basement
room for customers who want to put carpet over the
radiant slab. I'm concerned the carpet will insulate
off some of the heat, but I can't talk them out of it.
The slab is 4 to 5 inches thick, and the 1/2-inch PEX
tubing is laid out on 12-inch centers, with 11/2 inches
of styrofoam insulation under the slab and around its
edges. The slab is at least 5 feet below grade; local
frost depth is 48 inches. Is there a type of carpet and
pad that would allow more heat to radiate into the
room? Will the carpet cause heat to be lost into the
ground?

A.John Siegenthaler, a
consulting engineer specializing in hydronic
heating system design in Holland Patent, N.Y.,
responds: There are many successful radiant
heating installations where carpet is placed over a
concrete slab. For best performance, use a low-pile
commercial-grade level loop carpet bonded directly
to the top of the slab, which will provide low
thermal resistance to upward heat flow. If a pad
must be used, it should be a low-resistance slab
rubber pad approximately 1/4 inch thick, which will
add about 0.31 to the upward R-value of the carpet
(avoid polyurethane pads because of their higher
R-value). Given the tube spacing you have, and the
fact that basement heating loads are typically low,
adding the slab rubber pad will likely raise the
required circuit water temperature about
5¼F.